1. Field of the Invention
This invention relates to double safety valves of the type used in connection with machines such as presses, in which it is desired to disenable the machine should the pneumatic control system exhibit faulty behavior. The invention is particularly concerned with the construction of a monitor which senses discrepant pressure conditions between the two sides of the double safety valve and, in response to such a condition, disenables the supply circuitry to the solenoids which control the pilot valves.
2. Description of the Prior Art
It is known to provide double safety valves with an electro-pneumatic monitor in the form of a centered spool which shifts to operate an electrical lockout switch in response to discrepant pressures between the two valves. An example of this known monitor is shown on drawing 968C93 of the assignee of the present application. This prior art monitor has a relatively large spool engaged on opposite sides by centering springs. Restrictions in the housing passages leading from the sensing ports to the end chambers of the monitor were of some assistance in de-sensitizing the spool for minor pressure fluctuations. Although the prior construction provided acceptable and dependable service when the double valves operated at the most generally used supply pressure in the area of 60 PSI, undue sensitivity was sometimes exhibited where operating pressures were used which were relatively high. This could cause lockout of the monitor for minor main valve irregularities which would otherwise be considered normal. This type of lockout is called a "nuisance lockout."
One particular feature of the present invention, captive springs at opposite ends of the monitor spool, are in themselves known for a monitor spool of the type which controls the supply of fluid pressure rather than the electrical supply to the solenoids. This is shown for example in assignee's Drawing No. 309C90.